The biology core is responsible for in vivo and in vitro characterization and validation of new ligands and radiotracers prepared in this program project, for carrying out pharmacokinetic evaluation of the ligands for organ dosimetry calculations, and for assisting with the preparation of physician-sponsored Investigation of New Drug applications (IND's). Specifically, the biology core tests analogs of known receptor binding compounds using in vitro binding assays for receptor affinity, density, and specificity; performs in vitro receptor autoradiography with promising radioligands to determine their regional distribution; tests the radiolabeled analogs of known receptor ligands for their in vivo distribution and binding to various receptors in the brains of rodents and baboons, and compares them with their radiolabeled parent compounds (when available); validates pharmacologically the tracer as a ligand for a particular binding site; examines the in vivo metabolism of the radiotracers, and determines the whole body biodistribution in rodents to make radiation dose estimates which are necessary for human studies.